Two-dimensional electron gases (2DEGs)—narrow conducting channels at the surfaces and interfaces of semiconductor materials—are the bedrock of conventional electronics. The startling 2004 discovery that such 2DEGs could be engineered at the interface between two insulating transition-metal oxides, SrTiO3 and LaAlO3, initiated a worldwide effort to harness the functionality of oxide materials for advanced electronic applications. Now, an international collaboration working at the ALS has shown that the interface is not required. Using only intense synchrotron light, the group has been able to create and control 2DEGs at the bare surfaces of the insulating oxides SrTiO3 and KTaO3. As well as suggesting a potential methodology to spatially pattern 2DEGs in a wide variety of complex oxides, this discovery opens a new avenue for spectroscopic investigation of these novel electronic systems.

An Opening for Oxide Electronics

One usually thinks of metals like copper as electrical conductors and their oxides as insulators, as typified by electrodes in flashlights that work fine until they're corroded (oxidized). But in today's world of advanced materials, all such traditional rules have to be set aside. For example, transition metals can form a wide variety of oxides owing to an incomplete inner electron shell, and some of these exhibit a rich spectrum of properties, such as high-temperature superconductivity, magnetism, and ferroelectricity. What's more, they often exhibit much larger responses to external stimuli. But for a long time, it was thought that their chemical complexity would preclude their use in device applications.

All changed in 2004 with the discovery that the interface between insulating LaAlO3 and SrTiO3 could support a narrow conducting channel of the form utilized in virtually all semiconductor electronics. Now, Meevasana et al. have shown that even the interface is not required. Using only exposure to ultraviolet light, they demonstrated the creation of this type of conducting channel (called a two-dimensional electron gas or 2DEG) at the bare surfaces of two different transition-metal oxides. This opens up the possibility of creating 2DEGs in a much wider range of oxides than has been possible at interfaces. Detailed spectroscopic measurements of their electronic structure may hold the key to understanding the unique properties of transition-metal oxide 2DEGs and their potential for novel applications.

The collaborators were performing angle-resolved photoemission spectroscopy (ARPES) measurements at Beamline 10.0.1 on the insulating oxide SrTiO3. In an insulator, no electronic bands are expected close to the Fermi level, so it came as a surprise when the team clearly observed dispersive electronic bands intersecting the Fermi level at the surface of the SrTiO3, simply by cleaving the sample in vacuum and then exposing it to intense ultraviolet light. The measurements revealed the gradual formation of electronic states that are confined to a layer at the sample surface sufficiently thin that quantum-size effects emerged (quantum effects that arise when the length scale is comparable to the electron wavelength).

A multi-orbital two-dimensional electron gas created at the surface of transition-metal oxides, such as SrTiO3 and KTaO3, upon exposure to intense ultraviolet synchrotron light.

The creation of such a two-dimensional conducting channel at the bare surface of SrTiO3 shows that an interface with the polar surface of another oxide—thought to be a major factor driving 2DEG formation at SrTiO3/LaAlO3 interfaces—is not always a prerequisite for creating a two-dimensional electron system in a three-dimensional transition-metal oxide host. Relaxing this requirement opens the possibility of engineering 2DEGs in more exotic parent compounds. Indeed, the team has already demonstrated this potential with the formation of a 2DEG in the 5d transition-metal oxide KTaO3. This material is a compound whose large spin–orbit interactions suggest potential for exploiting the spin of the electron in the emerging variant of electronics termed "spintronics."

In SrTiO3, the 2DEG did not exist on the freshly cleaved sample, but its electron density could be precisely tuned by controlling the exposure to synchrotron ultraviolet light. This methodology opens a new route for fast and inexpensive patterning of the ground-state electron density of the 2DEG, thereby promising ways to combine novel semiconductor device architectures with the increased functionality of oxides and to scale these down to nanometer dimensions.

More than just a tool for monitoring evolution of the 2DEG density, the ARPES measurements allowed the collaborators to build up the first detailed spectroscopic picture of the electronic structure of complex oxide 2DEGs. They found that the 2DEGs are composed of multiple sub-bands of light carriers with low effective masses, which coexist with much heavier bands, reflecting an electronic structure derived from multiple orbitals. Moreover, the orbital degeneracies are strongly modified as compared to the bulk band structure, suggesting the delicate interplay of quantum confinement, multi-orbital physics, and spin–orbit interactions, which may help drive novel properties of the resulting 2DEGs, such as enhanced thermoelectric properties.

Multi-orbital sub-band structure of transition-metal oxide 2DEGs created at the surface of SrTiO3 and KTaO3. An orbital ordering is observed, where the bulk degeneracy of light- and heavy-mass bands is broken, in good agreement with tight-binding calculations.

Finally, the lightest bands observed have lower effective masses than the bulk bands from which they derive. Similar to 2DEGs in conventional semiconductors such as InAs, the rapid dispersion of these states in SrTiO3 indicates the presence of fast-moving electrons, which likely give rise to the high mobilities observed in interfacial oxide 2DEGs. At the same time, the researchers also observe spectroscopic signatures of strong electronic correlations. Such interactions typically lead to heavier masses. The unusual co-existence of fast-moving, yet strongly interacting, electrons could hold the key to engineering novel functionality into a new generation of all-oxide electronic devices.

ARPES measurements of a 2DEG in SrTiO3 and in the conventional semiconductor InAs. Both show fast-moving light electrons, but in SrTiO3 there are additional signatures of strong interactions, such as the long tail of spectral intensity extending well below the band bottom.

Funding: U.S. Department of Energy (DOE), Office of Basic Energy Sciences (BES); UK Engineering and Physical Sciences Research Council; European Research Council; Scottish Funding Council; the Thailand Research Fund; Suranaree University of Technology; and the Japan Society for the Promotion of Science. Operation of the ALS is supported by DOE BES.